How do you know when to use 'mal-'?
From what I can tell, 'mal-' is a prefix that can give an adjective a negative or opposite meaning, eg. bela "beautiful" -> malbela "ugly". But can 'mal-' only be used with certain adjectives, or can it be added to anything?
Like in English, the prefixes 'un-', 'in-', etc. can only be used with certain words; is it the same sort of thing here?
And if 'mal-' can be added to any adjective, could you (for example) attach it to the word "ugly" to then mean "beautiful"? So, bela "beautiful" -> malbela "ugly", and [ugly] "ugly" -> mal[ugly] "beautiful". (I guess what I mean here is, can 'mal-' be used both ways?)
Sorry if these are really obvious questions, I'm completely new to Esperanto. And I'm not good at explaining things (especially not when it's gone midnight lol) so sorry if this is unclear.
I find the book "Being Colloquial in Esperanto" by David K. Jordan to be very useful for answering these types of questions. The book is available online: http://pages.ucsd.edu/~dkjordan/eo/colloq/colloq.html.
In particular, checkout the explanation under the heading "mal" here: http://pages.ucsd.edu/~dkjordan/eo/colloq/colloq130.html#sec13-1
So I am fairly new to Esperanto as well. But I do know that in addition to the "mal-" words (malvarma, malbela, malbona, etc) there are also a few cases where Esperanto has two separate words to talk about opposite concepts. For example war is "milito" and peace is "paco". Those are the words you'll find in the dictionary.
However, unlike most languages, you could also use "malpaco" and "malmilito" to communicate the same idea of war and peace. Even if they aren't necessarily the "proper words", their meaning is clear and Esperantists will understand you.
So to answer your questions, yes I believe you can add mal- to any adjective and use it "both ways" as you say it. That is the great power of the mal- prefix. :)
Exactly. English requires people to learn sooooo much vocabulary. To imply the word “not” in an English word, there are the prefixes: un-, non-, dis-, dif-, mal-, anti-, de-, in-, im-, -il, -ir, a-, an-, and some other prefixes that I haven't mentioned. It is redundant to say the least.
Ok, so as far as I've seen you can stick mal- onto just about any adjective to give it the opposite meaning. For your second part, because of the way Esperanto is constructed there's only one root for adjective pairs, bela and malbela in your example, so cases where you'd be able to go backwards and forwards are very unlikely exceptions rather than common cases.
Some people use these words, but they are not sanctioned by the Akademio de Esperanto. So if you intend to use EO as an interlingvo, those words are more or less deprecated. I realize there is a school of thought that people should just do whatever they please without regard for the principles (of simplicity and limited redundancy) the language is built on. Or that certain words have become common enough, no matter how superfluous they are, to be used freely. But even if that is a valid view, I don't think new learners should be encouraged to use them without an explanation of their status. Not saying there is anything wrong with using them, but it is a good moment to explain how Esperanto is "guided" and how each user should approach his responsibility for keeping it intelligible.
You can use mal- on everything. I am not joking. Malmanĝi, malpost, malmatene, mal"saluton". If it follows the rules, it is correct, even if the word you created does not make any sense (yet). Ekzemple, mi scias, ke malbaleno estas la malo de baleno. A good Esperantist should be able to understand a word you created in the go. Duolingo is sadly not one of them because of the limitation of its code. Be creative.
Yes, malbaleno is an "opposite-whale," or presumably an "opposite of a whale." This might be a clever or poetic usage in the right context. However, I would probably use "malo de baleno", to be clear. Why? Because when people get carried away with mal- they see opposites where, strictly, none exist. Is red the opposite of green because they are on opposite sides of a color wheel? Malverda! Is infinity the opposite of zero? Creativity is highly encouraged, but remember that what works in your head may not be obvious to others. If you want to be understood in everyday speech, you should be somewhat conservative with mal-.
I will answer you with a strange example : using the universal inverting prefix mal- (which derives from Russian and general Slavic malo, lesser, contrary to, and not from Latin and romance malum or mal meaniing evil or wrong) with colours. Why is green "verda" and not "malruĝa" (unred), why is blue "blua" and not "malflava" (unyellow), why is black "❤❤❤❤❤" and not "malblanka" (unwhite) in the way as dark is "malhela", the opposite of "hela" (light)? Actually the opposite of white is not black, it is all possible dark colours most opposite to white depending on the object, it might even be dark red when speaking of wine : Homer in his Odysseus used the greek word "mehlas" which can mean black and also any rich dark colour proper to each object : ripe grapes, deep sea water, sun-tanned skin, , old dark wood, old red wine were are "malblankaj" ("unwhite", "mehlas" in Greek) for Homer. In the same way "malruĝa" when it applies for example to skin, doesn't mean green (though it may mean greenish or bluish), it means pallid, wan, most opposite to "blushing red". "Malblua" when applying to the sky doesn't mean "yellow" (actually the bluer the sky the more yellow the sun especially at sunset) which would theoretically be the opposite in photography but for opticians and photographs only, it means "most cloudy, overcast, grey". "Malverda" doesn't mean "red" as per photography or painting, it most probably means in nature devoid of vegetation, desert-coloured : grey, brown or auburn. "Malflava" means actually any possible cold and rather sad colour from turquoise to dark blue and dark grey : ancient Greek had another generic word for that : glaukos, the absolutely sunless and warmthless colour of sea depths and of witches' eyes, dark green blue of many hues. All depends on the object for which the absent colour prefixed by mal- indicates normality. Mal indicates not the mere absence of something but its total absence when it should most normally be expected at least to some moderate degree. Ne estas arboj ĉi-tie means that there are no trees, but malestas arboj ĉi tie means that trees are totally lacking, after a devastation for instance, where there should at least be some. Malmilito as the opposite of war is not paco, peace. Malmilito can be the refusal of any fight when there should be some to combat injustice, it can mean desertion, cowardness as well of the kind that will beget far more mortal wars in the future. Some adjectives denote a certain quality that is unidimensional : the opposite of high is clearly low no matter the object, no matter the figurative sence, like for instance with a price. But some other qualities like colurs are pluri-dimensional and their opposite is not a single direction but a whole region of reality.